Composition for forming lens, and Fresnel lens and transmission screen using said composition

ABSTRACT

The invention discloses a composition for forming lens, which contains urethane acrylate prepolymer based on tolylenediisocyanate as a principal material, and further said composition containing brome acrylate at least by 10 to 40 weight %, caprolactone denatured hydroxypivalic acid ester neopentylglycoldiacrylate by 3 to 20 weight %, and bis-(acryloxyethoxy)bisphenol A by 3 to 60 weight %, and refractive index of said cured product being 1.5 or more, further discloses a Fresnel lens using said composition, and a transmission screen 1, in which a Fresnel lens 3 is formed on a base material 2.

FIELD OF THE INVENTION

The present invention relates to a composition for forming a lens and toa Fresnel lens and a transmission screen of rear projection type usingsaid composition, and in particular to a transmission screen having aFresnel lens of short focal length.

BACKGROUND ART

For a transmission screen to be used in microfilm reader, projectiontelevision, etc., attempts are made to increase light diffusion propertyof the screen. For this purpose, a lenticular lens, Fresnel lens, etc.are provided on light source side, and a light shielding layer isfurnished on observer side to enhance screen contrast, and a lightdiffusion substance is added on the screen to increase light diffusionproperty of the screen.

Conventionally, to form such transmission screen, a synthetic resinsheet is produced by extrusion molding method, press molding method orcasting method, and this is laminated with film of base material, and alenticular lens or a Fresnel lens is formed directly on the film.

In the transmission screen, a Fresnel lens is arranged between varioustypes of light sources such as cathode ray tube, liquid crystal filtertype projection system on one side and a lenticular lens sheet on theother side, and light beam from the light source entering at high angletoward the surrounding region of the screen is converged slightlyinwardly to prevent the darkening of the surrounding regions of thescreen due to divergence of light from the light source.

When light beam enters Fresnel lens, refractive angle of the beam isdetermined according to the material of the Fresnel lens. The higherrefractive index the material has, the less the light beam is lost evenwhen the beam is refracted at high angle. Thus, if a material withhigher refractive index is used for Fresnel lens, it is possible torefract light beam entering at high incident angle with less reflectingloss. As the result, it is possible to bring the position of the lightsource much closer to the screen and to shorten the distance between thelight source of projection television and the screen. This contributesto designing of the projection television in smaller size.

Then, it is necessary to use a lens of short focal length as the Fresnellens in order to shorten the distance between the light source of animage and the transmission screen. For this purpose, instead of methylmetacrylate resin with high transparency (refractive index n=1.49), amaterial of higher refractive index such as styrene is copolymerizedwith methyl metacrylate, and a sheet of the resultant material (e.g.having refractive index n=1.57) is used for the purpose by moldingthrough thermal pressing.

However, the productivity of such Fresnel lens is not very high becauseit is produced by a series of processes including heating, pressurizingand cooling. In particular, to mold a Fresnel lens having fine lensshape over a large area by thermal pressing, it is necessary to heatunder strict temperature condition and to perform pressurizing andcooling for long time, and this leads to very low production efficiency.

Under such circumstances, it has been proposed to utilize a UV-settingresin, which is cured by ionizing radiation such as ultraviolet raywithin short time, instead of thermosetting resin as conventionally usedin order to improve production efficiency and to form lenses with fineshape.

The UV-setting resin has a prepolymer of UV-setting resin as itsmaterial. The prepolymer of UV-setting resin is a macromolecularsubstance with low polymerization degree and high viscosity, and it isnot possible to coat it on or to fill into a die. For this reason, it isnecessary to use a diluent to dissolve the prepolymer. If an organicsolvent is used as the diluent, the molded object is deformed due toevaporation of the organic solvent, and it is practiced to use a monomeras the diluent to polymerize with the prepolymer.

For a composition of UV-setting resin to be used in Fresnel lens ortransmission screen using such lens must have, along with transparencyof cured resin, high toughness not easily susceptible to rubbing as wellas adequate surface hardness against deformation. As UV-setting resinusable for such purpose, there is urethane type acrylate resin. In orderthat the resin cured by ultraviolet ray maintains high refractive index,various types of acrylate resin effective to improve density of theresin are used in the monomer serving as diluent. However, therefractive index of a composition containing acrylate is not verysatisfactory when compared with a material for thermal molding, in whicha material with high refractive index such as styrene and methylmetacrylate are polymerized.

Also, in a two-sheet type transmission screen, it is practiced tocombine a sheet where a Fresnel lens is formed with a sheet where alenticular lens is formed, and further to transport the combined sheetand to incorporate it in the equipment for projection television.However, there is a problem that Fresnel lens surface is brought intocontact with and damaged by surface of the other lens sheet due tovibration during transportation. In the past, such damage has beenprevented by placing a buffer sheet between lens sheets or by adding alubricant such as silicon oil. However, when the buffer sheet is placed,it is necessary to remove the buffer sheet by taking away the Fresnellens sheet and the lenticular lens sheet. When lubricant such assilicone oil is used, the cost is increased and the resultant imagebecomes uneven due to uneven application of lubricant.

With the circumstances as such, there have been strong demands on acomposition of UV-setting resin for transmission screen, which is madeof a resin, having high refractive index, high transparency of curedresin, and high toughness against damage caused by rubbing.

DISCLOSURE OF THE INVENTION

The composition for forming a lens according to the present inventioncomprises a composition of UV-setting resin and uses urethane acrylateprepolymer based on tolylenediisocyanate (TDI) given by the formula (1)below as a urethane type acrylate resin which is characterized bycolorlessness, transparency and flexibility as a prepolymer ofUV-setting resin and by low-vulnerability to damages caused by rubbingof the screen, further using 2-functional acrylate having benzene ringsuch as bis(acryloxyethoxy)bisphenol A given by the formula (2) as areactive diluent, and brome acrylate is added to increase refractiveindex. Further, to improve flexibility and increase anti-frictionproperty, caprolactone denatured hydroxypivalic acid esterneopentyldiacrylate is added. As the result, a Fresnel lens can beobtained, which maintains high refractive index and high anti-frictionproperty.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmission screen having a Fresnel lens, which isformed on a base material of synthetic resin, using a composition forforming lens of the present invention;

FIGS. 2(a)-2(c) represent drawings for explaining a method formanufacturing a lens using a composition for forming lens of the presentinvention; and

FIGS. 3(a)-3(d) show drawings for explaining another method formanufacturing a lens using a composition for forming lens of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Giving an example of brome acrylate usable in the present invention,acrylate such as ethyleneoxide addition product of tetrabromobisphenol Ais used in addition to tribromophenol methacrylate such as1-methylacrylate-2,4,6-tribromophenol given by the following formula(3). Brome acrylate is contained in the composition by 10 to 40 weight%.

The more the brome acrylate is added, the higher refractive index theFresnel lens has. However, the maximum content of brome acrylate is 40weight %, because it is in solid state at normal temperature anddissolved in the composition is limited. Also, it must be contained bymore than 10 weight % because it is difficult to obtain sufficientrefractive index if it is contained by less than 10 weight %. ##STR2##

As the urethane type acrylate, there is urethane type acrylateprepolymer based on isophoronediisocyanate (IPDI), given by the formula(4) below, in addition to urethane acrylate based ontolylenediisocyanate. When IPDI is used as prepolymer, cured resin hasgood flexibility and toughness. ##STR3##

However, the urethane type UV-setting resin primarily based onisophoronediisocyanate does not have sufficient refractive index. Incontrast, tolylenediisocyanate to be used as a composition for forminglens of the present invention has a benzene ring unlikeisophoronediisocyanate which has a 6-member ring. For this reason, itcan provide a resin of higher density compared withisophoronediisocyanate and contributes to the improvement of refractiveindex.

To obtain urethane acrylate type prepolymer to be used in a compositionfor forming lens of the present invention, tolylene-diisocyanate isbrought into reaction with polyol components such as polyether polyol,polyester polyol, etc., and a prepolymer having acrylate at the terminalis used. It is contained in the composition of the present invention by10-60 weight %.

Caprolactone denatured hydroxypivalic acid esterneopentylglycoldiacrylate is a 2-functional acrylate, which is producedby esterizing hydroxypivalic acid ester neopentylglycol withcaprolactonediol to have a longer straight chain in the skeleton, andflexibility is increased by lowering density of functional group. Thisdiacrylate is contained by 3 to 40 weight %. When the content of thisdiacrylate is lower than 3 weight %, sufficient flexibility cannot beobtained. If it is more than 20 weight %, refractive index is decreased.

As the caprolactone degenerated hydroxypivalic acid esterneopentylglycoldiacrylate to be used in the present invention, HX-220(trade name) manufactured by Nippon Kayaku Co., Ltd. may be used.

As the monomer to be used as a reactive diluent for the prepolymer ofurethane type acrylate, it is necessary to use the one, which maintainshigh refractive index when curing. From such viewpoint, 2-functionaldiacrylate of bisphenol A having a benzene ring on main chain is used.

Bisphenol A type diacrylate is contained by 3 to 60 weight %. When thecontent of bisphenol A type diacrylate is lower than 3 weight %,dilution property is not sufficient. If it is more than 60 weight %,moldability is worsened by excessive fluidity of the resin due to lowerviscosity when the cured resin is molded.

To merely increase the density, it is preferable to use mono-functionalepoxyacrylate. However, the molded product of mono-functional epoxyacrylate becomes too hard after curing and it adheres well to the othersubstances. Thus, when the IV-setting resin composition thus obtained isapplied to or filled into a die for manufacturing Fresnel lens, itadheres too tightly to the die, and it is difficult to separate it fromthe die. In contrast, in case of multi-functional acrylate, cured resinis softer and does not easily adhere to the die. Thus, it can be used asa reactive diluent with high refractive index and good mold releasingproperty.

Bis-(acryloxyethoxy)-bisphenol A is a diacrylate, which can be obtainedby adding acrylic acid to hydroxyl group at the end of ethyleneoxide orpropyleneoxide addition product of bisphenol A, and added molar numberof ethyleneoxide or propyleneoxide gives influence on flexibility andrefractive index of the cured resin. In other words, long chain ofethyleneoxide or polypropyleneoxide contained in main chain of thepolymer improves flexibility of the cured product, while the number ofbenzene rings in the cured product is decreased, and the refractiveindex is lowered.

According to the present invention, it is possible to keep good balanceof flexibility and high refractive index by simultaneously usingbis-(acryloxyethoxy)-bisphenol A with ethyleneoxide with smaller addedmolar number and the one with higher added molar number. For thispurpose, it is preferable to contain the former by 1.5 to 30 weight %,and the latter by 1.5 to 30 weight %.

As the bis-(acryloxyethoxy)-bisphenol A to be used in the presentinvention, FM-300 (trade name) manufactured by Nippon Kayaku Co., Ltd.as the one with ethyleneoxide with smaller added molar number, andBR-800 (trade name) manufactured by Nippon Kayaku Co., Ltd. as the onewith higher added molar number.

In the present invention, as a reactive diluent, mono-functionalurethane acrylate monomer having hydroxyl group and benzene ring may becontained by 3 to 20 weight %. The mono-functional acrylate increasesflexibility of the cured product and contributes to the improvement ofrefractive index. As mono-functional urethane acrylate monomer to beused in the present invention, OPP-2 (trade name) manufactured by NipponKayaku Co., Ltd. may be used.

Further, it is preferable that the above composition containingpentaerythritol type 4-functional monomer by 1 to 4 weight % is used asthe composition for forming lens of the present invention.Pentaerythritol type 4-functional monomer contributes to crosslinkingwhen curing and increases elasticity of the cured product. When thecontent of pentaerythritol type 4-functional monomer is lower than 1weight %, sufficient elasticity is not obtained. If it is contained bymore than 4 weight %, cured product becomes too hard due to excessivecrosslinking.

As pentaerythritol type 4-functional monomer to be used in the presentinvention, RP-1040 (trade name) manufactured by Nippon Kayaku Co., Ltd.may be used.

As the reactive diluent, a substance other thanbis-(acryloxyethoxy)-bisphenol A may be added to the above compositionin such quantity as not to affect the other physical property. Forexample, a monomer such as phenoxyethylacrylate may be used.

The composition for forming lens according to the present inventioncontains urethane acrylate based on tolylenediisocyanate as principalcomponent. Accordingly, a number of benzene rings are contained in thepolymer,i.e. cured product, and the cured product may be turned toyellowish due to oxidation of benzene rings. Therefore, it is preferableto add amine type additive to the composition of the present inventionby 0.3 to 2 weight %. Amine type additive can stabilize double bondingin benzene rings by its reduction effect and improve weatherproofproperty.

As the amine type additive to be used in the present invention, IM-820(trade name) manufactured by Nippon Kayaku Co., Ltd. may be used.

In the composition for forming lens according to the present invention,compositions are blended so that the refractive index after curingbecomes 1.5 or more. When refractive index of the cured product is lowerthan 1.5, the prepared Fresnel lens sheet does not provide a lens withsufficiently short focal length.

The composition for forming lens according to the present invention canbe cured by irradiation of ultraviolet ray or ionizing radiation such aselectron beam. When it is cured by ultraviolet ray, photopolymerizationinitiator such as sensitizer is added. As the photopolymerizationinitiator, acetophenones, benzophenones, thioxanthones, amines, etc. canbe used. For example, Dalocure 1173, Dalocure 1116, Dalocure 953(Merck), Bycure 55 (Stoffer), Ilgacure 184, Ilgacure 500, Ilgacure 651(Ciba Geigy) may be used. To the composition of the present invention,light diffusing agent, antistatic agent, coloring agent, surface activeagent, flame retardant, mold releasing agent, slipping agent, defoamingagent, softening agent, etc. can be added.

FIG. 1 shows a transmission screen having a Fresnel lens formed by thecomposition for forming lens according to the present invention. In atransmission screen 1, a Fresnel lens formed on a base material 2 ofsynthetic resin is combined with a lenticular lens sheet 5. In thetransmission screen of the present invention, Fresnel lens is arrangedon light source side to prevent darkening of the screen due todivergence of light in the surrounding region.

FIGS. 2(a)-2(c) show cross-sectional views of an embodiment formanufacturing a lens using the composition for forming lens according tothe present invention. As shown in FIG. 2 (a), the composition forforming lens was placed on one end of a lens forming mold 21 bysqueezing method, flow coating method or roll coating method to build upa resin pool 22 of the composition. Then, as shown in FIG. 2 (b), a basematerial 23 for a transparent lens sheet, which a primer layer 24 wasformed, laminated on the lens forming mold 21 with the resin pool 22 ofthe composition. From the region where the resin pool was furnished, apressure roll 25 was rolled over and was pressed on the base material,pushing air bubbles out. At the same time as pressurizing, ultravioletray was irradiated from a ultraviolet ray source 26, and the compositionfor forming lens was cured. After curing, as shown in FIG. 2 (c), themold was separated, and a Fresnel lens sheet 28 with a Fresnel lens 27formed on it was prepared. The Fresnel lens sheet thus prepared did notcontain air bubbles in its lens unit.

As the base material for the lens sheet, transparent synthetic resinsheet, film, etc. such as polymethylmethacrylate, polystyrene,polycarbonate, etc. may be used. As the primer layer, a material havinggood adhesive property to both the base material for lens sheet and thecomposition for forming lens according to the present invention may beused. For example, vinyl chloride-vinyl acetate copolymer type primer,butyral type primer, urethane type primer, phenoxy type primer, etc. maybe used.

FIGS. 3(a)-(d) represent another method for manufacturing a lens usingthe composition for forming lens according to the present invention. Asshown in FIG. 3 (a), the composition for forming lens was diluted by asolvent to decrease viscosity, and it was used to form a first resinlayer 32 on a mold 31 by silk screen method. Then, organic solvent wasevaporated, and a resin pool 33 of the composition for forming lens wasfurnished on the first resin layer at one end of the mold. Then, asshown in FIG. 3 (c), a base material 35 consisting of transparentacrylate resin, on which a primer layer 34 consisting of vinylchloride-vinyl acetate copolymer was formed, was laminated. From theregion where the resin pool was furnished, a pressure roll 36 was rolledover and it was pressed on the base material, pushing air bubbles out.At the same time as pressurizing, ultraviolet ray was irradiated from anultraviolet ray source 37 to harden the composition for forming lens. Asshown in FIG. 3 (d), the mold was separated, and a lens sheet 39 havinga Fresnel lens 38 was obtained. The Fresnel lens prepared by thismanufacturing method has good form reproducibility and is advantageousin that less air bubbles are intermingled than the method describedabove.

The composition for forming lens according to the present invention hasUV-setting prepolymer based on tolylenediisocyanate as main material. Itis a UV-setting resin composition containing brome acrylate at least by10 to 40 weight %, caprolactone degenerated hydroxypivalic acid esterneopentylglycoldiacrylate by 3 to 20 weight %, andbis-(acryloxyethoxy)-bisphenol A by 3 to 60 weight %. The resin obtainedby curing through ultraviolet ray is colorless and transparent, hasflexibility and is not vulnerable to rubbing, and has high refractiveindex. If this composition is used in Fresnel lens, it is possible toobtain a Fresnel lens with short focal length.

In the following, description will be given on some examples:

(EXAMPLE 1)

On a die, a UV-setting resin composition was coated, which had beenprepared by mixing UV-setting prepolymer based on tolylenediisocyanate(Iu011(s); Nippon Kayaku Co., ltd.) by 25 weight %,bis-(acryloxyethoxy)-bisphenol A (FM-300; Nippon Kayaku Co., Ltd.) by 50weight %, and 1-methylacrylate-2,4,6-tribromophenol by 25 weight %.

On the layer of the UV-setting resin, a sheet of acryl resin (SumipexHT; Sumitomo Chemical Industry Co., ltd.) of 3 mm in thickness waspressed and laminated so that air bubbles are not intermingled. From thedirection of acryl resin sheet, ultraviolet ray was irradiated for 20seconds from a high voltage mercury lamp, and the UV-setting resincomposition was cured.

When refractive index of the lens thus prepared was measured by Abberefractometer, it was 1.57 and was equal to the refractive index 1.57 ofthe material, which was prepared through copolymerization of styrene andpolymethylmethacrylate.

(EXAMPLE 2)

The composition for forming lens primarily based on urethane acrylateprepolymer was coated on a chromium-plated die of Fresnel lens, using adispenser. An acryl plate (Sumipex HT; Sumitomo Chemical Industry Co.,Ltd.) of 3 mm in thickness where a primer layer made of vinylchloride-vinyl acetatecopolymer (VAGH (trade name); Union Carbide:) waspressed and laminated on the resin filled in the above die so that theair is not intermingled. Ultraviolet ray was irradiated for 30 secondsusing high voltage mercury lamp to cure the composition for forminglens. Then, the die was separated, and a Fresnel lens sheet wasprepared.

The lens thus prepared had refractive index of 1.55.

    ______________________________________                                        [Chemical composition of the composition for forming                          lens]                                                                         ______________________________________                                        Urethane acrylate prepolymer (trade name                                                               37.0 weight %                                        TU03; Nippon Kayaku Co., Ltd.) based on                                       tolylenediisocyanate                                                          Mono-functional urethane acrylate monomer                                                               9.0 weight %                                        (trade name OPP-2; Nippon Kayaku Co., Ltd.)                                   Bis-(acryloxyethoxy)-bisphenol A (Nippon                                      Kayaku Co., Ltd.)                                                             FM-300 (trade name)      10.0 weight %                                        BR-800 (trade name)      10.0 weight %                                        Brome acrylate (Nippon Kayaku Co., Ltd.)                                                               20.0 weight %                                        BR-100 (trade name)                                                           Pentaerythritol type 4-functional monomer                                                               2.0 weight %                                        (trade name RP-1040; Nippon Kayaku Co., Ltd.)                                 Caprolactone denatured pivalic acid ester neo-                                                         10.0 weight %                                        pentylglycoldiacrylate (trade name HX-220;                                    Nippon Kayaku Co., Ltd.)                                                      1-hydroxycyclophenoxylphenylketone                                                                      0.7 weight %                                        (trade name Ilgacure 184; Ciba Geigy)                                         Amine type additive       1.0 weight %                                        (trade name IM-820; Nippon Kayaku Co., Ltd.)                                  Fluorine type surface active agent                                                                      0.3 weight %                                        (trade name Florade FC-430; Sumitomo 3M Co.,                                  Ltd.)                                                                         ______________________________________                                    

INDUSTRIAL APPLICABILITY

The resin prepared by curing the composition for forming lens accordingto the present invention by ultraviolet ray is colorless andtransparent, has flexibility and is not vulnerable to rubbing and hashigh refractive index. Accordingly, when a Fresnel lens is manufacturedfrom this resin, it is possible to obtain a lens with shorter focallength. When a transmission screen having the Fresnel lens between alight source and a lenticular lens is used for a projection television,it is possible to shorten the distance between the light source and thetransmission screen because it has short focal length. This contributesto compact design of the system.

What we claim is:
 1. A transmission screen, in which a Fresnel lens islaminated with a base material, said Fresnel lens being formed by acomposition for forming lens, containing UV-setting prepolymer based ontolylenediisocyanate as principal material, and further containing bromeacrylate by 10 to 40 weight %.
 2. A transmission screen, in which aFresnel lens is laminated on a base material, said Fresnel lens beingformed by a composition for forming a lens according to claim 1, whereinsaid composition further comprises caprolactone denatured hydroxypivalicacid ester neopentylglycoldiacrylate by 3 to 20 weight %.
 3. Atransmission screen, in which a Fresnel lens is laminated on a basematerial, said Fresnel lens being formed by a composition for forming alens according to claim 1 or 2, wherein said composition furthercomprises a reactive diluent of bis(acryloxyethoxy)-bisphenol A by 3 to60 weight %.
 4. A transmission screen, in which a Fresnel lens islaminated on a base material, said Fresnel lens being formed by acomposition for forming a lens according to claim 1, wherein saidcomposition further comprises pentaerythritol 4-functional monomer by 1to 4 weight %.
 5. A transmission screen, in which a Fresnel lens islaminated on a base material, said Fresnel lens being formed by acomposition for forming a lens according to claim 2, wherein saidcomposition further comprises pentaerythritol 4-functional monomer by 1to 4 weight %.
 6. A transmission screen, in which a Fresnel lens islaminated on a base material, said Fresnel lens being formed by acomposition for forming a lens according to claim 3, wherein saidcomposition further comprises pentaerythritol 4-functional monomer by 1to 4 weight %.
 7. A method of forming a transmission screen, in which aFresnel lens is laminated on a base material, said Fresnel lens beingformed by the steps of:providing a UV-setting prepolymer resincomposition based upon tolylenediisocyanate, which contains a bromeacrylate compound of 10 to 40 weight % and a reactive diluent of 3 to 60weight %, supplying said resin composition to a lens forming mold, andcuring said resin composition in the mold by UV irradiation.
 8. A methodof forming a transmission screen according to claim 7, wherein thereactive diluent provided is bis-(acryloxyethoxy)-bisphenol A.
 9. Amethod of forming a transmission screen according to any one of claims 7to 8 further comprising providing said resin composition withcaprolactone denatured hydroxypivalic acid esterneopentylglycoldiacrylate of 3 to 20 weight %.
 10. A method of forming atransmission screen according to claim 9 further comprising providingsaid resin composition with pentaerythritol 4-functional monomer of 1 to4 weight %.